Modern communication systems employ various modulation formats to transmit information over one or more carriers. Accordingly, performance of such systems may be sensitive to the used modulation format.
Digitally modulated signals are used to transport high-speed data, video and voice on cable networks. The high-speed signals are subject to a variety of impairments that can seriously impact the quality and reliability of the services being provided. Operators and technicians responsible for the performance of a cable network have to be able to measure the high-speed signals on their network in order to understand how well the system is working and to determine the root cause of degradation to the delivery of services to their customers. In order to measure a signal, current test equipment requires the user to enter certain parameters in order to define the signal to be measured.
Two-way hybrid fiber-coaxial (HFC) networks are shared bi-directional networks with point-to-multipoint transmission in the downstream direction, and multipoint-to-point transmission in the upstream direction. Signals are distributed via a fiber optic connection from a head-end to a node that converts the optical signal to an electrical signal, and then distributes the signals to residences via a tree and branch coaxial cable distribution network. At the subscriber side, terminal equipment supports the delivery of cable services (video, data and voice services) to subscribers, via cable modems. Data and voice services are supported by cable modems and communication gateways, respectively, which require the use of an upstream signal path. The network uses a fiber optic upstream signal path from the node to the head-end. A return band is used to support transmissions from devices at subscribers' premises back to the head-end. In such networks, many cable modems may compete for communication bandwidth in both the upstream and downstream directions.
Delivery of data services over cable networks, and in particular cable television (CATV) networks, is typically compliant with a Data Over Cable Service Interface Specifications (DOCSIS®) standard. The term ‘DOCSIS’ generally refers to a group of specifications published by CableLabs that define industry standards for cable headend equipment, such as Cable Modem Termination System (CMTS), and cable modem (CM) equipment. Subscribers send data from their digital devices, such as personal computers (PC), VoIP phones, Video IP devices, etc, into the CM, which then relays the data to the CMTS, which in turn relays the information to an appropriate network element. Information destined to the subscriber digital device is provided from the network to the CMTS, which in turn relays the information to the CM. The CM in turn relays the information to the subscriber's digital device. The communication direction from the CMTS to the CM is referred to as the downstream direction, while the communication direction from the CM to the CMTS is referred to as the upstream direction.
The physical layer specification of DOCSIS provides for the use of frequency multiplexing and several specific forms of quadrature amplitude modulation (QAM) for both upstream (CM to headend) and downstream (headend to CM) communications. At the moment of writing, the physical layer specification of the most current version of the DOCSIS standard, DOCSIS 3.0, provides for a normal downstream operating range from 50 MHz to 1002 MHz, with either 6 MHz or 8 MHz spacing for downstream channels, which utilize 64-QAM or 256-QAM modulation format. The upstream operating frequency range may be between 5 and 42 MHz, or 5 to 85 MHz. The upstream channel widths are configurable and may take a set of define values be between 200 kHz and 6.4 MHz, each corresponding to a specific symbol rate, with the upstream data modulated with either QPSK, 8-QAM, 16-QAM, 32-QAM, 64-QAM or 128-QAM.
One technical challenge in operating a network having a bidirectional communication path on a shared medium between the headend and each remote point is maintaining network integrity for upstream and downstream signals. Noise and other undesirable energy originating at one remote point or at any point along the return path from that remote point can impair network communications for all remote points in the network. Similarly, where noise and undesirable energy from one remote point is combined with noise and or other RF impairments from other remote points in the network, network communications may be impaired. RF impairments may occur in many forms including, but not limited to, impulse and/or burst noise, common path distortion, and ingress such as interference from radio communication and navigation signals. Impulse noise or burst noise typically consists of high-power, short-duration energy pulses. Ingress is unwanted RF energy that enters the communication path from a source external to the communication path. Ingress often comprises radio and/or navigational communication signals propagated over the air that enter a weak point in a wireline network, although it may also comprise impulse and/or burst noise that is similarly propagated over the air to enter the network at a weak point. Weak points in the network often occur where there is a cable shield discontinuity, improperly grounded electrical device, or a faulty connector at or near a remote point. When radio frequency carriers from shortwave radio, citizen's band radio, or other broadcast sources enter the network at these weak points, they may cause interference peaks at specific carrier frequencies in the communication path. Common path distortion may be the result of second and higher order mixing products from the downstream channel that couple to the upstream channel; such nonlinear mixing may occur, for example, when physical electromechanical connectors corrode and oxidize, creating point contact diodes.
Therefore, an ability to monitor the performance of the cable network and to quickly and efficiently isolate impairments in the cable network is essential for the cable network operation. MSO's (Multiple System Operators) do not have a simple way to determine margin available before impairments present in their plant will trigger degradation in HFC (Hybrid Fiber Coaxial) plant metrics commonly associated with indicating noticeable degradation in subscriber services. They also do not have a way of predicting whether carriers will perform acceptably at higher-order modulations without actually turning up the higher order modulation and potentially disrupting subscriber's services in the process.
It is therefore an object of the present invention to provide a method and apparatus for network testing that overcome at least some of the disadvantages and limitations of the prior art by utilizing more informative symbol-level performance metrics and/or metrics predictive of the network performance for different modulation formats.